1. Field of Invention
The present invention relates to a surface acoustic wave device using crystal glass. More specifically, the invention relates to a surface acoustic wave device using a so-calledin-plane rotational ST-cut crystal plate, and a communications apparatus using the same.
2. Description of Related Art
Surface acoustic wave devices represented by surface acoustic wave resonators and surface acoustic wave filters utilizing a surface acoustic wave (SAW) use a planar piezoelectric material. Known surface acoustic wave devices have an IDT (Interdigital Transducer) electrode on the main surface of a piezoelectric flat plate and has reflectors on both sides of the IDT electrode to oscillate a high frequency region stably. ST-cut surface acoustic wave devices are known in which an ST-cut crystal plate is used as a piezoelectric material that forms a surface acoustic wave device for the purpose of reducing the fluctuation in frequency relative to temperature changes and in which the X-axis (electrical axis) of the ST-cut crystal plate is the propagating direction of the surface acoustic wave.
FIG. 2 is an explanatory diagram of an SAW resonator, which is an example of the surface acoustic wave device, wherein (1) is a plan view thereof and (2) is a partly cutaway sectional view taken along line A—A of (1). As shown in FIG. 2(1), an SAW resonator 10 uses, for example, an ST-cut crystal plate 12 as a substrate, having an IDT electrode 14 at the center of the main surface of the substrate. The IDT electrode 14 can be composed of a comb-like positive electrode 14a and a negative electrode 14b, wherein one comb-like electrode piece is arranged between the other comb-like electrode piece. The SAW resonator 10 applies high-frequency voltage to the positive electrode 14a and the negative electrode 14b of the IDT electrode 14 to generate surface acoustic waves.
Also, reflectors 16 (16a and 16b) having a plurality of short-circuit electrodes are disposed on both sides of the IDT electrode 14. The reflectors 16 reflect the surface acoustic waves toward the IDT electrode 14. For example, Japanese unexamined patent application publication number JP9-148879 discloses that a ratio of the electrode width to the pitch of electrodes can be set to 0.65 or more in order to reduce the frequency fluctuation due to the formation error of the IDT electrode. More specifically, it is disclosed that η≧0.65 must be satisfied where the ratio of an electrode width Lt and an interelectrode pitch Pt, which are shown in FIG. 2(2), is η. Reference symbol Ht shown in this drawing denotes the thickness (film thickness) of the IDT electrode 14.